Why is retinoblastoma restricted to the eye?

I know that this form of cancer is caused by a mutation in the Rb gene. But isn't it all of our cells contain the same set of genes. Why then, does only the Rb gene in the eye get mutated and develop into cancer?

The Rb gene is actually mutated in many human cancers, acting as a break to cell division in cells throughout the body. However, cancer is not only caused by mutations in single genes such as Rb, but other genes need to be mutated as well. In retinoblastoma for some unknown reason, you need Rb mutations, and likely some other changes, but you don't need to accumulate many many successive mutations to develop a tumor. In a form of retinoblastoma that runs in families, individuals when cured of their retinoblastoma are still at higher risk for other cancers, such as osteosarcoma later in life. It is true that the retina is extremely sensitive to cancer when this gene is mutated, more so than other cancers and this is still not well understood.

There are many, many different types of cells within the body. If our genes are the blueprints within our own bodies that initially created the cells which became the eye, it makes sense that there can be a disease which can distinguish between the cells of the eye too.